Potassium channels are involved in a number of physiological processes, including regulation of heartbeat, dilation of arteries, release of insulin, excitability of nerve cells, and regulation of renal electrolyte transport. Potassium channels are thus found in a wide variety of animal cells such as nervous, muscular, glandular, immune, reproductive, and epithelial tissue. These channels allow the flow of potassium in and/or out of the cell under certain conditions. For example, the outward flow of potassium ions upon opening of these channels makes the interior of the cell more negative, counteracting depolarizing voltages applied to the cell. These channels are regulated, e.g., by calcium sensitivity, voltage-gating, second messengers, extracellular ligands, and ATP-sensitivity.
Potassium channels are made by alpha subunits that fall into 8 families, based on predicted structural and functional similarities (Wei et al., Neuropharmacology 35(7):805–829 (1997)). Three of these families (Kv, Eag-related, and KQT, now referred to as KCNQ) share a common motif of six transmembrane domains and are primarily gated by voltage. Two other families, CNG and SK/IK, also contain this motif but are gated by cyclic nucleotides and calcium, respectively. The three other families of potassium channel alpha subunits have distinct patterns of transmembrane domains: inward rectifier potassium channels, Slo potassium channels, and TP potassium channels. Slo family potassium channels (also known as BK or “maxi” channels) are large conductance channel types, are voltage gated, have six to seven transmembrane domains, a pore loop domain, and a cytoplasmic tail domain involved in gating, e.g., ion (e.g., calcium) and pH regulation (see, e.g., Schreiber et al., J. Biol. Chem. 273:3509–3515 (1998); Butler et al., Science 261:221–224 (1993); Meera et al., Proc. Natl. Acad. Sci. U.S.A. 94(25):14066–71 (1997); Wei et al., Neuron 13:671–681 (1994)). The inward rectifier family of potassium channels (Kir), belong to a structural family containing 2 transmembrane domains (see, e.g., Lagrutta et al., Jpn. Heart. J. 37:651–660 1996)). Yet another functionally diverse family (TP, or “two-pore”) contains 2 tandem repeats of this inward rectifier motif.
As described above, potassium channels are typically formed by four alpha subunits, and can be homomeric (made of identical alpha subunits) or heteromeric (made of two or more distinct types of alpha subunits). In addition, potassium channels have often been found to contain additional, structurally distinct auxiliary, or beta, subunits (e.g., Kv, Slo, and KCNQ potassium channel families; see, e.g., McManus et al., Neuron14:645–650 (1995); Schopperle et al., Neuron 20:565–573 (1998); Brenner et al., J. Biol Chem. 275:6453–6461 (1999); and WO 0050444). These beta subunits do not form potassium channels themselves, but instead they act as auxiliary subunits to modify the functional properties of channels formed by alpha subunits. For example, the Kv beta subunits are cytoplasmic and are known to increase the surface expression of Kv channels and/or modify inactivation kinetics of the channel (Heinemann et al., J. Physiol. 493:625–633 (1996); Shi et al., Neuron 16(4):843–852 (1996)). In another example, the KCNQ family beta subunit, minK, primarily changes activation kinetics (Sanguinetti et al., Nature 384:80–83 (1996)).
The Slo family of potassium channels can be further divided into two subfamilies, based on homology. The first subfamily includes Slo1 and Slo3 (see, e.g., Elkins et al., Proc. Nat'l Acad. Sci USA 83:8415 (1986); Atkinson et al., Science 253:551 (1991); Adelman et al., Neuron 9:209 (1992) (Drosophila Slo1); Bulter et al., Science261:221–224 (1993); Dworetsky et al., Mol Brain Res. 27:189–193 (1994); Tseng-Crank et al., Neuron 13:1315–1330 (1994); McCobb et al., Am. J. Physiol 269:H767–H777 (1995); Wallner et al., Rec. Chan. 3:185–199 (1995) (human and mouse Slo1); Schreiber et al., J. Biol. Chem. 273:3509–3515 (1998); WO 99/20754 (human and mouse Slo3). Slo1 is calcium activated, while Slo 3 is regulated by internal pH. Potassium channels from the second subfamily include C. elegans Slo2 and rat “SLACK” (Joiner et al., Nat. Neurosci. 1:462–469 (1998) (rat SLACK or Slo2); Yuan et al., Nat. Neurosci. 3:771–779 (2000); Lim et al., Gene 240:35–43 (1999) (C. elegans “Slo2”). The members of the second subfamily share the same structural motifs, are also voltage gated, and can also be gated by other ions, e.g., calcium or chloride (see, e.g., Joiner et al., supra, Yuan et al., supra). However, the members of the second subfamily appear to share less overall homology to Slo1 and Slo3 channels.
Slo channels play a role in a wide variety of physiological processes ranging from renal salt secretion (Wang et al., Annu. Rev. Physiol. 59:413–36 (1997), regulation of neuronal and glandular secretion (Lingle et al, Ion Channels 4:261–301 (1996); Robitaille et al., Neuron 11:645–655 (1993); Peterson et al., Nature 307:693–696 (1984); Robitaille & Charlton, J. Neurosci. 12:297–305 (1992), sensory perception (Ramanthan et al., Science 283:215–217 (1999); Navaratnam et al., Neuron 5:1077–1085 (1997)) regulation of smooth muscle tone (Brayden & Nelson, Science 256:532–535 (1992)) and control of neuronal excitability (Knaus et al., J. Neurosci. 16:955–963 (1996); Robitaille & Charlton, J. Neurosci. 12:297–305 (1992); Lancaster et al., J. Neurosci. 11:23–30 (1991); Robitaille et al., Neuron 11:645–655 (1993)).